One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by thermal chemical vapor deposition (CVD) or a plasma chemical vapor deposition process, among other techniques. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Smaller feature sizes have resulted in the presence of increased aspect ratio gaps for some applications, for example, between adjacent conductive lines or in etched trenches. The aspect ratio of a gap is defined by the ratio of the gap's height or depth to its width. These spaces are difficult to fill using conventional CVD methods. A film's ability to completely fill such gaps is referred to as the film's “gapfilling” ability. Silicon oxide is one type of insulation film that is commonly used to fill the gaps in intermetal dielectric (IMD) applications, premetal dielectric (PMD) applications and shallow trench isolation (STI) applications among others. Such a silicon oxide film is often referred to as a gapfill film or a gapfill layer.
Some integrated circuit manufacturers have turned to the use of high density plasma CVD (HDP-CVD) systems to deposit silicon oxide gapfill layers. HDP-CVD systems form a plasma that is approximately two orders of magnitude or greater than the density of a standard, capacitively-coupled plasma CVD system. Examples of HDP-CVD systems include inductively-coupled plasma systems and electron cyclotron resonance (ECR) plasma systems among others. HDP-CVD systems generally operate at lower pressure ranges than low density plasma systems. The low chamber pressure employed in HDP-CVD systems provides active species having a long mean-free-path and reduced angular distribution. These factors, in combination with the plasma's density, contribute to a significant number of constituents from the plasma reaching even the deepest portions of closely spaced gaps, providing a film with improved gapfill capabilities as compared to films deposited in a low density plasma CVD system.
Another factor that allows films deposited by HDP-CVD techniques to have improved gapfill characteristics as compared to films deposited by other CVD techniques is the occurrence of sputtering, promoted by the plasma's high density, simultaneous with film deposition. The sputtering element of HDP deposition slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability of IIDP deposited films. Some HDP-CVD systems introduce argon or a similar heavy inert gas to further promote the sputtering effect. These HDP-CVD systems typically employ an electrode within the substrate support pedestal that enables the creation of an electric field to bias the plasma toward the substrate. The electric field can be applied throughout the HDP deposition process to generate sputtering and provide better gapfill characteristics for a given film. One HDP-CVD process commonly used to deposit a silicon oxide film forms a plasma from a process gas that includes silane (SiH4), molecular oxygen (O2) and argon (Ar).
However, a limitation associated with sputtering is an angular redistribution of sputtered material. For example, in an STI gapfill, the sputtered SiO2 can be sputtered from above the trench and deposit on the sides of the trench, causing excess buildup, and limiting the opening through which bottom-up gapfill is achieved. If there is too much re-deposition, the trench can close off before the bottom is filled, leaving a buried void within the trench.
FIG. 1A shows the initial stages of film deposition over a substrate (not shown) having a gap 120 defined by two adjacent features 122, 124 formed over the substrate. As shown in FIG. 1A, the conventional IIDP-CVD silicon oxide deposition process results in direct silicon oxide deposition on horizontal surface 126 within gap 120 and horizontal surfaces 128 above features 122, 124. The process also results in indirect deposition (referred to as re-deposition) of silicon oxide on sidewalls 130 due to the recombination of material sputtered from the silicon oxide film as it grows. In certain small-width, high-aspect-ratio applications, the continued growth of the silicon oxide film results in formations 132 on the upper section gap sidewall that grow toward each other at a rate of growth exceeding the rate at which the film grows laterally on lower portions 134 of the sidewall (see FIG. 1B also). The final result of this process is that a void 136 forms as shown in FIG. 1C.
With tighter trenches, the risk of closing the trench before the bottom-up fill is complete becomes greater, even with the lighter atoms. The He process has been able to provide a gapfill solution down to 110 nm and the H2 process has extended HDP-CVD down to 65 nm. Spin-on dielectric and ALD (atomic layer deposition) films have shown gapfill capabilities down to much tighter structures than HDP-CVD has been able to fill, but customers are somewhat reluctant to make the switch for other reasons such as film shrinkage and reliability.
Another method to keep the top of the trench open and allow the bottom-up fill to be completed is to use a dry NF3-based plasma etch at the point of the process where the trench has nearly closed, as described in U.S. Pat. No. 6,908,862, which is hereby incorporated by reference for all purposes. The NF3 dissociates in the plasma, forming reactive fluorine radicals. These radicals are able to break the Si—O bond in the deposited film, forming SiF4, a volatile species, according to:4F.+SiO2(s)→SiF4(g)↑+O2(g).The SiF4 is pumped out along with the excess O2, removing the deposited film and opening up the trench. This Dep-Etch-Dep (“DED”) process provides a small but significant margin to the original process in terms of gapfill.
Generally, in a conventional DED process, each deposition step uses different process conditions optimized for the characteristics of the substrate prior to the deposition step. For example, the initial deposition step “dep1” may deposit a relatively thin layer, intended to protect the sidewalls of the trench from the subsequent etching step “etch1”. The second deposition step “dep2” typically includes deposition of a relatively thicker layer.
If the DED includes further deposition and etching steps, then each individual deposition step (dep1, dep2, a third deposition “dep3,” and so on) and each individual etch step (etch1, a second etch “etch2,” and so on) is typically designed with conditions optimized specifically for that step. For example, dep2 is typically optimized based on the profile of the layer at the completion of etch1, dep3 is optimized based on the profile of the layer at the completion of etch2, and so on.
A disadvantage of conventional DED processes with separately tuned processes for each deposition and etch step is that each step generally needs to be separately qualified in manufacturing. In other words, before installing a DED process in a production manufacturing line, a number of substrates are generally run on dep1 to qualify dep1, a number of substrates are generally run on etch1 to qualify etch1, and so on.